Using timers like that won't account for hour changes or any other adjustments that may me made to the system clock after the timer is set.
Also, mIRC's timers aren't entirely accurate. For intervals in the range of a couple of hours that's fine, over the course of a week it could be noticeable. I'm not sure if the inaccuracy from each week would tend to drift in either direction effectively limiting the size of the error or whether it would be cumulative and get very wrong very quick, but in any case there's a good chance it won't be triggering right on midnight each week.
Using the timer's built-in time parameter is a lot simpler and will eliminate these problems.
